Chronic stress makes people more susceptible to cancer and Alzheimer's disease. It can accelerate aging and hurt cognitive function.

In other words, living under a constant cloud of stress can do a lot of damage.

But temporary bursts of moderate stress may, in fact, be good for you, research is starting to show. Several recent studies from Bay Area scientists have found that a short, sharp spike in nerves - even just the anxiety of a pending math test - can actually have protective, positive biological effects.

"It's really the way Mother Nature intended us to use our stress responses," said Firdaus Dhabhar, an associate professor of psychiatry at Stanford.

Most of the research about stress has looked at chronic stress. But researchers say it is logical that some stress is beneficial. Exercise is a stressor, for example, and its benefits are well known. And our basic fight-or-flight instincts - another common form of acute stress - help us survive.

But scientists are just beginning to tease apart what exactly the benefits of short-term stress are and what is occurring on the molecular and cellular levels.

Dhabhar has found that when a temporary stressor - even the natural anxiety before a medical procedure - is coupled with an event that triggers an immune response, such as a surgery or vaccine injection, then the immune response is enhanced.

"The stress can come from any source," Dhabhar said.

In one study from 2009, Dhabhar and other researchers found that people undergoing knee surgery whose moderate stress levels before the operation activated immune cells recovered faster and more completely in the first year than patients who had lower immune responses.

Cellular protection

A study published in March by a team of UCSF and Stanford researchers found that a bit of stress neutralized some of the harmful molecules that build up in cells and protected cells from what is known as oxidative damage.

The study examined a group of women under chronic stress from taking care of a spouse or parent with dementia, and compared them to women with low levels of stress. The participants had to talk about themselves and perform math in front of a group of people who intentionally did not give them a warm reception.

Overall, the added strain caused greater cellular damage in the chronically stressed women than in the control group of non-stressed women. But among women in the group who had low stress to begin with, those who felt the most anxious about the test beforehand saw "significantly reduced levels of oxidative damage" in their cells, according to the study.

"Our best guess is that we may see certain types of mild stress can up-regulate antioxidant defenses, and now we are hoping to follow up to see if this is indeed the case," said Kirstin Aschbacher, an assistant professor of psychiatry at UCSF and an author of the study.

More human studies

Some of the acute stress research is limited by the fact that the studies often are performed in animals, primarily because the field is still very new. Still, researchers hope that the findings gleaned from the animal models will be able to be reproduced in studies with human participants.

A UC Berkeley study published last month, for example, showed that brief periods of stress can enhance mental performance in rats.

In the study, the rats were immobilized for a few hours, which temporarily elevated levels of the stress hormone corticosterone, the rat version of the human hormone cortisol. Researchers found that the increase in corticosterone induced stem cells to generate new nerve cells in the brain's hippocampus, an area associated with memory.

It took two weeks for the new nerve cells to mature, but once they did, the rats performed better on a memory test than they had on the same test before they were stressed.

The new nerve cells "were part of the neuronal network," said Daniela Kaufer, an associate professor of neuroscience at UC Berkeley. "We could show they were specifically activated."

Promoting good stress

One focus of future research will be looking at how to maximize the effects of those short bursts of stress, Dhabhar said.

Scientists have wondered if manipulating stress hormone levels or putting someone through a psychological test could spark the same health benefits seen after naturally stressful situations.

And while experts said that finding ways to reduce overall stress and the duration of it is a vital determinant of health, there are likely more benefits to be discovered about a little stress here and there.

"Not all stress is bad - there's no need to get stressed about stress," Dhabhar said. "Admittedly, it's easier said than done."